1. Field of the Invention
The present invention relates to a method for displaying images, including characters and color backgrounds based on image data stored in an image memory of an image processing device
2. Description of the Prior Art
A display method called a bit map method is often applied to an image processing device which is designed to make an arbitrary layout of characters on a display unit possible. This method stores character patterns in a memory, regards character patterns as images, and displays the character patterns based on the data stored in the memory. Therefore, by this method, sizes and display positions of the characters can be changed as required during the procedure of storing the data of the character patterns.
Character patterns obtained by the bit map method can be synthesized with a color background, like a picture, and displayed as an image using one of the following two methods.
(i) Store the character data and color background data separately, and synthesize both types of data. PA1 (ii) Develop the character data and the background data in one memory and output the data as a piece of an image. PA1 (a) Information on color backgrounds; PA1 (b) Information on characters; and PA1 (c) Flag for discriminating characters from a background.
Because the method (i) requires a memory for storing the character data, besides a memory for storing the color background data, the method (i) requires larger memory capacity than the method (ii) does. However, because the character data can be processed irrespective of the background data, processings including that of changing only the color of the characters are readily done even after synthesis of the character patterns has been completed. A conventional image processing device adopting this method (i) is shown in FIG. 3.
The device shown in FIG. 3 is comprised of an image memory 1 which includes a first memory area for storing n-bit data of color backgrounds and a second memory area for storing one-bit data of characters; a synthesizer 2 for synthesizing the data of the color background and the data of the characters supplied by the image memory 1; a display unit 3 and a micro processing unit 4 for controlling the above units, setting the image memory 1, etc.
Further, in the case where a number of complicated characters, such as Chinese characters, are required to be displayed on a display unit which has poor image to pixel decomposition few pixels are allocated to each character. Therefore, characters displayed on such a display unit only using black and white, as in the case of FIG. 3, are illegible.
A method expressing characters as images having different levels of gray has been proposed to solve the above mentioned problem. This method will now be described by taking a line pattern as an example as shown by FIGS. 4(a) and 4(b). In FIG. 4(a), each pixel of a line pattern is expressed using either black or white, and in FIG. 4(b), colors having different gray levels are allotted to pixels of the line pattern. When compared with the image of FIG. 4(a) which has remarkable indentations, the image of FIG. 4(b) has a much better quality, having no indentations.
This method, however, needs much a larger data quantity. For example, when the gray scale includes eight levels, three bits are needed to represent each dot of a character, which is three times as large as that required in the case of a binary image. Furthermore, if such character information having several gray levels, is stored separately from the color background information in accordance with the aforementioned method (i), memory capacity must be large. For example, assuming that k bits are used to represent each pixel of the characters, memory capacity is increased as shown in FIG. 5.